apt/cache.py, apt/package.py: Fixed PEP8 and pyflakes issues

This commit removed the unused local variable 'transient' in 'apt/cache.py'
and the unused import 'warnings' in 'apt/package.py'.
make pep8 test part of the unittests
add pyflakes test
add new pep8,pyflakes dependencies
add .travis.yml & update python3 dependency to 3.3

This patch adds support for the travis-ci.org CI system.

The "C" environment in .travis.yml is used to avoid having PPA
python versions installed in the travis chroot.

python3.3 is needed so that test_paths.py runs

The tests_paths.py will only work with python3.3 as it uses
the u"" prefix
make pyflakes clean
make test_pep8.py pass
Merge remote-tracking branch 'upstream/debian/sid' into feature/travis-and-pep8-pyflakes